Alternative approach to the generation of a unique response to a challenge

A device 30, which may comprise a physical unclonable function (PUF), generates a unique response to a challenge, the device comprising a plurality of structures 32 which can change from a first to a second distinct state in response to an appropriate challenge. The structures may be a series array of resonant tunnelling diodes (RTDs), but may also be other elements exhibiting negative differential resistance. The device is arranged to facilitate a challenge to the plurality of structures in combination to cause each structure to change from the first to the second state. The device is arranged to facilitate a measurement of an output of the structures in combination 34 in response to the input. The unique response is indicative of a sequence 36 in which the state change takes place for each structure. The device may exhibit non-linear behaviour such that a sweep-down of input voltages between two voltage values will not produce a reversed mirror-image switching sequence to that produced by a sweep-up between the same values. The device may comprise first and second separable parts, wherein one part is secured, the parts possibly being a vehicle lock and key or a battery and connector.